The blast furnace has been in successful operation in the steel industry for over a century. Recently, the basic design of the blast furnace has been adapted for use in the gasification of coal, especially when operating with a "slagging bottom" in which the mineral components of the coal ash are removed from the gasifier in the form of a molten slag. Still more recently, a shaft furnace having blast furnace geometry has been adapted for the simultaneous pyrolysis and gasification of briquetted mixtures of coal and cellulosic waste materials such as municipal solid waste, dewatered sewage sludge, and sylvan waste or shredded pulpwood. See U.S. Pat. Nos. 4,052,173 4,152,119 and 4,225,457. The preparation of the burden in the form of sturdy briquettes has made it possible to employ gasifiers of much larger diameter than the 13-foot diameter coal gasifiers that have thus far been proposed. The slagging Lurgi gasifier operated by the British Gas Corporation in Westfield, Scotland is less than 7 feet in diameter.
One problem that has limited the use of larger and more economical gasifiers is the difficulty of securing even distribution of the oxidizing medium when employing tuyeres that enter the gasifier from its periphery either horizontally or slightly canted. It is an object of the present invention to overcome this limitation.
Another problem that is especially acute when processing briquetted mixtures of coal and municipal solid waste is corrosion and erosion of the refractory lining by a molten slag that is constantly changing in composition and fluxing characteristics because of the heterogeneous and ever-changing nature of the inorganic impurities introduced with municipal solid waste and sewage sludge. The slag has a tendency to penetrate, impair and erode the refractory lining in the high temperature hearth area of the gasifier. It has been accepted practice to attempt to shield the refractory lining against such attack by providing a temperature gradient that would cause frozen slag to deposit on the inner surface of the refractory lining. This theoretically logical solution has failed in practice because it was found that the congealed slag had a tendency to crack and flake off with the result that the refractory lining is intermittently exposed to molten slag with resultant impairment of the refractory.
The foregoing difficulty is overcome in the present invention by providing a honeycomb structure of open cells or recesses which serve to support the islands of congealed slag that are caused to form within the recesses by imposition of a controlled temperature gradient. The ceramic retaining walls of the cells prevent the propagation of cracks and effectively eliminate the spalling that has traditionally defeated the practical application of the otherwise sound principle of interposing a barrier of frozen slag between the refractory lining and the corrosive molten slag.
Another problem that has frequently interfered with the smooth operation of a slagging bottom gasifier is related to the continuous removal of molten slag from the bottom of the gasifier without seriously eroding the tap hole or plugging the opening with frozen slag. The hearth design of this invention effectively precludes these operating difficulties, while providing for the continuous removal of the slag into a quenching vessel from which the water quenched granules are removed through a lock hopper. This design permits continuous slag removal when operating the gasifier either at atmospheric or at elevated pressures.
The operation of moving burden gasifiers has normally resulted in the production of gas contaminated with tars, oils and particulates. One of the objectives of the present invention is to produce a product gas that is free of these undesirable constituents.